A rotation sensing system disclosed in Japanese Laid-Open (Kokai) Patent Application No. 3067/86 comprises a rotation sensor in the form of an electromagnetic pickup or a Hall element. The rotation sensor outputs sinusoidal pulses in accordance with the rotation of an object to be sensed. Each of the sinusoidal pulses from the rotation sensor is shaped into a rectangular pulse by a waveform shaping circuit including a comparator. Although not clearly described in the above Japanese Laid-Open application, generally, the rectangular pulses are fed to a control portion having a microcomputer. In this control portion, the input rectangular pulses are processed so as to obtain information on the rotation of the object to be sensed.
The rotation sensing system described in the above Japanese Laid-Open Patent Application No. 3067/86 further comprises a circuit for detecting a malfunction of the rotation sensor. This malfunction detection circuit basically comprises a low-pass filter, and a window comparator. A voltage signal from the rotation sensor is fed to the low-pass filter, so that only the direct current (DC) component of this signal is outputted from the low-pass filter. In the window comparator, this DC component is compared with reference voltages. For example, when the rotation sensor is subjected to a wire breaking (malfunction), the voltage level of the DC component exceeds one of the reference voltages, so that a malfunction detection signal of a high level is outputted from the window comparator.
Japanese Laid-Open Patent Application No. 155950/75 also discloses a malfunction detection circuit comprising a low-pass filter and a comparator.
The rotation sensing systems disclosed in the above Japanese Laid-Open Patent Application Nos. 155950/75 and 3067/86 are disadvantageous in that the construction of the circuitry is complicated because of the need for the addition of the malfunction detection circuit comprising the low-pass filter and the comparator.
U.S. Pat. No. 4,825,691 discloses a rotation sensing system which comprises a pair of rotation sensors for sensing the rotations of two objects to be sensed, respectively, which objects are rotated in associated relation to each other, each of the rotation sensors being in the form of an electromagnetic pickup. In this system, as in the above-mentioned systems, sinusoidal pulses outputted respectively from the two rotation sensors are shaped into respective rectangular pulses by respective waveform shaping circuits each comprising a comparator, and then are inputted into a microcomputer. In the microcomputer, the inputted rectangular pulses are processed so as to obtain information on the rotations of the two objects to be sensed.
The microcomputer of this system has the function of detecting a malfunction. More specifically, when the rectangular pulse from one of the rotation sensors is not inputted into the microcomputer even if the rectangular pulse from the other rotation sensor is inputted into the microcomputer, it is judged that the rotation sensor is subjected to a malfunction.
The system disclosed in the above U. S. patent, however, requires the plurality of rotation sensors for the purpose of the malfunction detection, and therefore this technique can not be applied to a system having only one rotation sensor. Moreover, when both of the two rotation sensors are subjected to malfunction at the same time, the rectangular pulses from these sensors are not inputted into the microcomputer. This condition is the same as the condition in which the rotations of the objects to be sensed are stopped. Therefore, it is not judged whether the former condition represents a malfunction or the stopping of the rotations of the objects to be sensed.